Fins for attachment to a swimmer's feet are well known. Usually the foot fins are aligned with the swimmer's feet, that is, at right angles to his legs, to facilitate walking on dry ground but when worn in the water the feet must be stretched to a position approximately parallel to the legs and thereafter the legs are kicked up and down in the water, with the swimmer's body being in a substantially prone position. Retaining the feet unnaturally stretched while kicking against the resisting inertia of the water at the extremities of the legs can be extremely tiring. This problem has been recognized and a solution has been suggested in the patent to Crowder No. 3,078,482 wherein propulsion units are disposed along the legs rather than only at their extremities thus making use of leg strength where it is greatest. In that patent a symmetrical flexible hydrofoil, similar in cross-section to a double-camber airplane wing, is supported between two flat, parallel rigid plates spaced apart laterally and interconnected by metallic rod-like members which pass through each hydrofoil adjacent its front and rear edges. As the swimmer kicks his legs, the hydrofoil flexes between the rod-like members with one side of the hydrofoil being concave and the other convex depending on the direction of kick. During a kick, the hydrofoil and both plates partially rotate in unison against the restoring force of a torsion spring coaxial with the forward rod-like member and though it is said the camber of the foil aids forward propulsion it is obvious that as a leg approaches the limit of movement of a kick, the then concave side of the foil acts as a scoop resisting completion of a kick and also resisting forward movement of the swimmer. In addition the plates, rods, and spring create turbulence and could be dangerous to another nearby swimmer. The Crowder unit required the rigid frame defined by the end plates and through-members in order to resist any tendency of the foils to simply fold back about their inner ends to positions substantially parallel to the swimmer's legs where they would be substantially useless for any purpose.